RULE OF THUMB
Q From Eric J Michelsen in the USA, Frank Conway in Winnipeg, Canada, and several others: I’d like to know what the term rule of thumb meant. I remember reading it had something to do with being permitted to beat your wife with a rod no thicker than your thumb. Is this correct?
A This sounds like the invention of somebody desperately trying to make sense of a traditional phrase — what linguists call folk etymology. And it’s quite certainly untrue. But there’s a lot more to it than just fevered imagination.
The expression rule of thumb has been recorded since 1692 and probably wasn’t new then. It meant then what it means now — some method or procedure that comes from practice or experience, without any formal basis. Some have tried to link it with brewing; in the days before thermometers, brewers were said to have gauged the temperature of the fermenting liquor with the thumb (just as mothers for generations have tested the temperature of the baby’s bath water with their elbows). This seems unlikely, as the thumb is not that sensitive and the range of temperatures for fermentation between too cool and too warm is quite small.
It is much more likely that it comes from the ancient use of bits of the body to make measurements. There were once many of these: the unit of the foot comes from pacing out dimensions; the distance from the tip of the nose to the outstretched fingers is about one yard; horse heights are still measured in hands (the width of the palm and closed thumb, now fixed at four inches); and so on. There was an old tailors’ axiom that “twice around the thumb is once around the wrist”, which turns up in Gulliver’s Travels. It’s most likely that the saying comes from the length of the first joint of the thumb, which is about an inch (I remember once seeing a carpenter actually make a rough measurement this way). So the phrase rule of thumb uses the word rule in the sense of ruler, not regulation, and directly refers to this method of measurement.
So where does beating your wife come in? Sharon Fenick wrote an article about its origins in the newsgroup alt.folklore.urban in 1996. She found that for more than two centuries there have been references in legal works to the idea that a man may legally beat his wife, provided that he used a stick no thicker than his thumb; but the references were always to what some people believed, not to established legal principle. The British common law had long held that it was legal for a man to chastise his wife in moderation, as one might a servant or child, but Sir William Blackstone wrote in his Commentaries on the Laws of England in 1765 that this principle was by then in decline. So far as I can discover nothing was ever laid down about how such discipline should be applied.
Ms Fenick traced the idea back to a pronouncement that was supposed to have been made in 1782 by a British judge, Sir Francis Buller; this led to a fiercely satirical cartoon by James Gillray that was published on 27 November that year, in which Buller was caricatured as Judge Thumb. (Buller was a brilliant lawyer, the youngest man ever to be appointed a judge in Britain, at 32, but he was widely considered hasty and prejudiced in his opinions.)
It might be that he never made the statement that rendered him so notorious. Edward Foss, in his Biographical Dictionary of the Judges of England of 1864 says that to Buller “is attributed the obnoxious and ungentlemanly dictum that a husband may beat his wife, so that the stick with which he administers the castigation is not thicker than his thumb”, but says he can’t find any evidence Buller said it. But the Dictionary of National Biography and other standard works say firmly he did, as did contemporary biographies.
However, it was only in 1976, so far as I can discover, that the traditional phrase rule of thumb became directly associated with this spurious legal maxim, through a bit of wordplay in a report that was misunderstood by readers.
It is extraordinary that we can so accurately pinpoint the moment at which this folk belief came into being. And how astonishing, too, that it should have survived more than two centuries to become part of the folklore of modern times.